A. Field of the Invention
The present invention relates to the field of scanning probe microscopes, including those which use light beam detection schemes.
B. Description of the Prior Art
Scanning force microscopes, also referred to as atomic force microscopes, can resolve features of matter to the atomic level, i.e., determine features measured to an accuracy of + or −0.10 Angstrom. Scanning force microscopes are members of a class of a broader category of microscope known as scanning probe microscopes. As is commonly known, scanning probe microscopes use a probe that senses some parameter of a sample such as height, or magnetic field strength. A sensor will typically monitor a parameter of the probe, such as vertical displacement. Scanning probe microscopes include scanning tunneling microscopes, scanning force microscopes, scanning capacitance microscopes, scanning thermal microscopes, and other types of probe microscopes, as is well known.
When used to image the topography of a sample, the scanning force microscope uses a finely pointed stylus to interact with a sample surface. Scanning force microscope are typically used to measure the topography of recording media, polished glass, deposited thin films, polished metals and silicon in preparation for integration into semi-conductors. A scanning mechanism in the microscope creates relative motion between the stylus and the sample surface. When a measurement of the interaction of the stylus and surface is made, the surface topography of the sample can be imaged in height as well as in the lateral dimensions. Other classes of probe microscopes may use different types of probes to measure sample features other than topography. For example, the interaction of a magnetic probe with the sample may create an image of the magnetic domains of the sample. Scanning tunneling microscopes use a conductor with a sharp point and a small bias voltage to sense a sample surface which is then used to form an image of charge density.
Scanning force microscopes typically have the stylus mounted orthogonally to the longer dimension of a cantilever. A cantilever is a lever constrained on one end with the other end free to move. The stylus is attached to the free end, and the cantilever will, therefore, deflect, or bend, when forces are applied to the stylus. In force microscopes the forces acting on the stylus are the result of the interaction of the stylus with the sample surface. The combination of a stylus, cantilever, and inseparable cantilever supporting elements is referred to as a probe assembly. The cantilever, as used in a scanning force microscope, typically has a very weak cantilever force constant, and deflects or bends noticeably when forces as small as one nanonewton are applied to the free end. Typical cantilever force constant values for such cantilevers are in the range of 0.01 N/m to 48N/m, where N is in Newtons and m is in meters. A detection mechanism is operatively connected to provide a signal proportional to cantilever deflection. This signal is then processed by a feedback loop to create a feedback signal. The feedback signal in turn drives a vertical drive mechanism. The vertical drive mechanism moves the fixed end of the cantilever toward and away from the sample surface. This vertical drive mechanism maintains the free end of the cantilever surface at a nearly constant bend angle, as detected by the detection mechanism, through movement of the probe assembly in proportion to the magnitude of the feedback signal.
During scanning operation, a lateral drive mechanism creates relative lateral motion between the stylus and sample. This relative lateral motion between the stylus and the surface creates lateral and vertical forces on the stylus as it interacts with surface features passing under the stylus. The lateral force applies torque to the stylus and cantilever. The vertical force on the stylus causes the cantilever free end to deflect vertically. The known lateral position of the stylus over the sample can be expressed in terms of x and y coordinates. The vertical deflection of the cantilever defines a height or z value. The x and y coordinates create a matrix of z values which describe the surface topography of the sample. The scanning mechanism includes the vertical and lateral drive mechanisms.
In order to detect and quantify the cantilever deflections, a laser beam is directed onto the free end of the cantilever opposite the surface supporting the stylus. The surface illuminated by the laser beam is at least partially reflecting. By measuring the position of the reflected beam, as the lateral drive mechanism operates, the deflection of the free end of the cantilever is determined. Preferably, a vertical array of two conventional light-sensitive devices detects the position of the reflected beam. These devices produce electrical signals that represent the bend angle of the free end of the cantilever. The difference between the two signals created by the two light-sensitive devices is a signal that is proportional to the displacement of the cantilever deflection in the vertical direction. Alternatively, and most preferred, by use of four light-sensitive devices in a quadrant array both the cantilever twist and cantilever vertical deflection can be measured. The vertical drive mechanism receives signals processed from the vertical component of the output of the light-sensitive devices.
In probe microscopes it is often necessary to replace the probe assembly. This may result from a blunted stylus tip typically caused by wear of, or by small particles that adhere to, the tip as it scans over the sample. Also, the stylus or the cantilever, or both can break, thus necessitating replacement of the probe assembly. When the probe assembly is replaced, the new cantilever often is not in the same position as the previous cantilever, relative to the laser and associated optics. Adjustment of either the laser beam angle or the probe assembly position is then required. Conventional alignment mechanisms restore the beam to its proper position on the reflecting surface of the cantilever.
The initial adjustment of the laser to direct its beam onto the cantilever can be accomplished in various ways. See, for example, U.S. Pat. No. 5,861,550, “Scanning Force Microscope and Method for Beam Detection and Alignment” by Ray, and U.S. Pat. No. 6,189,373, by Ray, titled “Scanning Force Microscope and Method for Beam Detection” and, U.S. Pat. No. 5,874,669 by Ray titled “Scanning Force Microscope with Removable Probe Illuminator Assembly”.
The manufacture of the probe together with its associated stylus, may be accomplished with micro-machining, ion beam milling, or other techniques as are well known. In some instances the resulting stylus may have an improper shape, such as, for example an aspect ratio or a nonsymetry that will prevent its use. When such a stylus is used to scan a sample surface, the image obtained would be distorted. Thus, before use, a stylus may be characterized by first scanning a sample of known surface features and then comparing the known features with the image obtained by the stylus. If the stylus has an undesirable shape the image will not compare favorably with the known sample features and the stylus typically will be rejected in favor of a stylus that provides a favorably comparable image. For the purpose of the present invention, and as is commonly understood in this field, the above-described process is known as stylus or tip characterization.
Known scanning probe microscopes are shown in U.S. Pat. No. 4,935,634 to Hansma et, al, and U.S. Pat. No. 5,144,833 to Amer et. Al. These devices move the sample laterally and vertically under a stationary stylus while detecting the cantilever deflection with the laser beam apparatus described above. These microscopes have a disadvantage stemming from the limited force capability of the lateral and vertical drive mechanisms. When the sample weight is great compared to the force created by the drive mechanisms, the sample will then move very slowly or not at all under the stylus. The mechanical resonance of these scanning mechanisms is also undesirably low with large moving mass.
Other known microscopes as described in U.S. Pat. No. 5,496,999 to Linker et. Al. and U.S. Pat. No. RE35,514 to Albrecht et. Al. have removable assemblies comprising laser, cantilever, and adjustment mechanisms mounted to the fixed reference frame of the microscope base. But, these microscopes also have the disadvantage as described above in that they move the sample under the stationary stylus. Further, the assemblies are too massive to be mounted to the lateral and vertical drive mechanisms because they permit adjustment of the beam path or probe position only while the assembly is mounted to the microscope.
Other known microscopes arc also described in U.S. Pat. No. 5,481,908 and its continuation U.S. Pat. No. 5,625,142 to Gamble. These microscopes use a stationary sample, but move the laser, the cantilever and all of the associated mechanisms necessary to make initial adjustment of the laser beam. Because the laser moves with the cantilever, the laser beam follows the motion of the cantilever during scanning. However, the relatively great mass of the moving part of these microscopes limits the rate of image data collection.
Other known microscopes attempt to overcome the disadvantage of moving the sample by using an interferometric method to track a moving cantilever. These microscopes are described in U.S. Pat. No. 5,025,658 and its continuation U.S. Pat. No. 5,189,906 to Elings et. Al. However, this approach suffers from false signals received by the interferometer as a result of light reflected from the sample surface.
Still other known microscopes use moving beam steering optics with a stationary laser source as described in U.S. Pat. Nos. 5,524,479 and U.S. Pat. No. 5,388,452 to Harp and Ray; U.S. Pat. No. 5,463,897 and U.S. Pat. No. 5,560,244 to Prater et. Al.; and in U.S. Pat. Nos. 5,440,920 and U.S. Pat. No. 5,587,523 to Jung et. Al. These microscopes employ a fixed position laser and optical elements that move in conjunction with the moving probe assembly. As a result of the moving optical elements, the laser beam experiences a refraction such that it more or less follows the reflecting surface of the moving cantilever. However, these microscopes have noticeable deficiencies when the probe assembly must be replaced because initial alignment of the laser beam through the optics and onto the newly installed cantilever are typically time consuming and tedious. As a result these microscopes do not readily lend themselves to industrial applications.
With these microscopes, it is possible to place a low mass operator controlled adjustment mechanism on the moving end of the drive mechanisms to reposition the probe assembly rather than aligning the laser. The probe assembly then can be aligned with the laser beam. However, the vertical and lateral drive mechanisms often consist of thin walled piezoelectric tubes, and such tubes are quite fragile. The operator may apply too much force when adjusting the probe holding mechanism attached to the tubes thus damaging or breaking the tubes during the alignment process. Also, this alignment process can also be tedious.
Other known attempts to solve this problem, such as described in U.S. Pat. No. 5,496,999 to Linker et. Al. use precision mounting of the probe assembly on the microscope. By carefully machining the parts to high tolerances it is possible to bring the probe into near alignment with the laser light source. This method, however, generally results in higher costs and normally still results in the need for a final small adjustment of the laser beam or probe position.
Still other attempts to solve this problem as exemplified in U.S. Pat. No. 5,705,814 rely on systems that move the scanning mechanism into a position relative to the probe assembly using an X,Y translator, a Z translator and an optical system to detect when the scanning mechanism and the to-be-mounted probe assembly are in alignment. This approach then uses either a vacuum or a mechanical mechanism to capture and hold the probe assembly. These systems are very complex and expensive relative to the invention presented in this application.